At present, China's photovoltaic power generation system is mainly a DC system, which is to charge the battery to the battery, and the battery directly supplies power to the load. For example, the solar household lighting system used in northwest China and the microwave station power supply system far away from the power grid are DC system. Such a system is simple in structure and low in cost, but due to different DC voltages (such as 12V, 24V, 48V, etc.), it is difficult to achieve standardization and compatibility of the system, especially for civil power, because most of the AC load is DC power. The photovoltaic power supply for power supply is difficult to enter the market as a commodity. In addition, photovoltaic power generation will eventually be connected to the grid. This requires a mature market model. In the future, the AC photovoltaic power generation system will become the mainstream of photovoltaic power generation.
Inverter requirements in the application:
1. Requires higher efficiency. Due to the current high price of solar cells, in order to maximize the use of solar cells and improve system efficiency, it is necessary to improve the efficiency of the inverter.
2. Requires high reliability. At present, photovoltaic power generation systems are mainly used in remote areas, and many power stations are unattended and maintained. This requires the inverter to have a reasonable circuit structure, strict component screening, and requires the inverter to have various protection functions, such as input DC. Polarity reverse protection, AC output short circuit protection, overheating, overload protection, etc.
3. The DC input voltage is required to have a wide adaptation range. Since the terminal voltage of the solar cell varies with the load and the intensity of the sunlight, the battery has an important effect on the voltage of the solar cell, but the voltage of the battery depends on the remaining capacity and internal resistance of the battery. The fluctuations of the change, especially when the battery ages, the range of the terminal voltage varies greatly. For example, for a 12V battery, the terminal voltage can vary between 10V and 16V, which requires the inverter to have a large DC input voltage. The range is guaranteed to work properly and the AC output voltage is stable.
4. In medium and large capacity photovoltaic power generation systems, the output of the inverter power supply should be a sine wave with less distortion. This is because in medium and large capacity systems, if square wave power is used, the output will contain more harmonic components, and higher harmonics will generate additional losses. Many photovoltaic power system loads are communication or instrumentation equipment. The equipment has high requirements on the quality of the power grid. When the large-capacity photovoltaic power generation system is connected to the grid, in order to avoid power pollution with the public power grid, the inverter is also required to output a sine wave current.
The inverter converts direct current into alternating current. If the direct current voltage is low, it is boosted by the alternating current transformer to obtain the standard alternating voltage and frequency. For large-capacity inverters, because the DC bus voltage is high, the AC output generally does not require transformer boost to reach 220V. In medium and small-capacity inverters, the DC voltage is low, such as 12V, 24V. It is necessary to design a boost circuit.
Medium and small capacity inverters generally have three types: push-pull inverter circuit, full-bridge inverter circuit and high-frequency boost inverter circuit. The push-pull circuit connects the neutral plug of the step-up transformer to the positive power supply, and the two power tubes. Alternate operation, the output obtains AC power, because the power transistor is connected in common, the driving and control circuit is simple, and because the transformer has a certain leakage inductance, the short-circuit current can be limited, thereby improving the reliability of the circuit. The disadvantage is that the utilization of the transformer is low and the ability to drive the inductive load is poor.
The full-bridge inverter circuit overcomes the shortcomings of the push-pull circuit. The power transistor adjusts the output pulse width, and the effective value of the output AC voltage changes accordingly. Since the circuit has a freewheeling circuit, the output voltage waveform is not distorted even for inductive loads. The disadvantage of this circuit is that the power transistors of the upper and lower arms are not common, so a dedicated drive circuit or an isolated power supply must be used. In addition, in order to prevent the common conduction between the upper and lower bridge arms, it is necessary to design a turn-on circuit after the turn-off, that is, the dead time must be set, and the circuit structure is complicated.
Inverter circuit control circuit
The main circuits of the above several inverters need to have a control circuit to realize. Generally, there are two control modes of square wave and positive weak wave. The square wave output inverter power supply circuit is simple, the cost is low, but the efficiency is low, and the harmonic components are large. . The sine wave output is the development trend of the inverter. With the development of microelectronic technology, a microprocessor with PWM function has also been introduced, so the inverter technology of sine wave output has matured.
1. Square wave output inverters currently use pulse width modulation integrated circuits, such as SG3525, TL494 and so on. Practice has proved that the use of SG3525 integrated circuit, and the use of power FET as a switching power component, can achieve higher performance and cost of the inverter, because SG3525 has the ability to directly drive the power FET and has an internal reference source and operational amplifier and Undervoltage protection, so its peripheral circuitry is simple.
2. Sinusoidal output inverter control integrated circuit, sine wave output inverter, its control circuit can be controlled by microprocessor, such as 80C196MC produced by INTEL, MP16 produced by Motorola, and MI-CROCHIP PIC16C73, etc., these MCUs have multi-channel PWM generators, and can set the dead time between the upper and upper arms. The circuit of sine wave output is realized by INTEL 80C196MC. 80C196MC completes the sine wave signal generation and detects AC output voltage for voltage regulation.
Inverter main circuit power device selection
The selection of the main power components of the inverter is crucial. The power components currently used are Darlington Power Transistors (BJT), Power FETs (MOS-FETs), Insulated Gate Transistors (IGBTs) and turn-offs. Thyristors (GTOs), etc., use more devices in small-capacity low-voltage systems as MOSFETs. Because MOSFETs have lower on-state voltage drop and higher switching frequency, IGBT modules are generally used in high-voltage and large-capacity systems. Because the MOSFET increases with the increase of voltage, the IGBT has a large advantage in the medium capacity system. In the large capacity (100kVA or higher) system, GTO is generally used as the power component. .
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